Method for Enhancing Current Throughput in an Electrochemical System

ABSTRACT

An electrochemical system with reduced limiting-current behavior is disclosed. The electrochemical system is useful for fuel cells and bio-sensors. In part, the invention relates a method of reducing or eliminating limiting-current behavior in the operation electrochemical systems, in particular those with ion-selective membrane or electrochemical electrodes, by spatially reducing the convection near the membrane or the electrode. The invention further relates to electrochemical systems in which micropores, microarrays or pillar arrays are used to reduce convection in comparison to conventional systems without microarrays, micropores or pillar arrays.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/355,703 filed Jun. 17, 2010. The entire teachings of theabove-referenced application are incorporated herein by reference.

GOVERNMENT SUPPORT

The invention was supported, in whole or in part, by Grant No.CBET-0854026; 6919872 from the National Science Foundation. TheGovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

Ion concentration polarization (ICP) is a fundamental electrochemicalphenomenon that describes the mismatch of charge carriers at thenanoporous membrane or nanochannel interface (Rubinstein, et al.,Journal of Chemical Society Faraday Transactions II, 75, 231 (1979);Holtzel, et al., Journal of Separation Science 30, 1398 (2007)).Significant concentration gradient can be generated when ion current istransported through perm-selective membranes (nanochannels) orelectrodes, altering the overall conduction properties of theseelectrochemical systems. Understanding ICP and related transportphenomena is important in many engineering fields such as bio-sensing,and fuel cells application. (Wang, et al., Anal. Chem., 77, 4293 (2005);Wang, et al., Lab Chip, 8, 392 (2008); Song, et al., Power Sources, 183,674 (2008)). Classical theory of ICP, originally developed by Nemst in1904, predicts that there will be a maximum current allowed through theperm-selective membrane (limiting current behavior), because ionconcentration in the anodic side of the membrane reaches near zero. Atthis point (called as limiting current condition) no further increase inion current through membrane is possible even if the bias is furtherincreased. (Probstein, et al., Physicochemical Hydrodynamics: AnIntroduction (Wiley-Interscience, 1994)). However, in reality,significant over-limiting current can always be observed experimentallyin most perm-selective membranes. The detailed physical mechanism ofover-limiting current has been in debate since as early as 1979.(Rubinstein, et al., supra). Overlimiting current is often associatedwith water dissociation at the vicinity of the membrane, but recently,Kim, et al. experimentally confirmed the existence of strong convectionof fluid layer near the membrane, in direct coincidence with theformation of depletion zone (diffusion layer) as well as the classicover-limiting current behavior. (Strathmann, et al., J. Membr. Sci.,125, 123 (1997); Kim, et al., Phys. Rev. Lett., 99, 044501 (2007)). Theimportance of fluid flow in ICP and limiting current behavior is nowwell established, both theoretically and experimentally. (Rubinstein, etal., Phys. Rev. E, 62, 2238 (2000); Pundik, et al., Phys. Rev. E, 72(2005); Rubinstein, et al., Phys. Rev. Lett., 101, 236101 (2008)).

Electrochemical systems using microfluidic technology are becomingimportant for a variety of fields including fuel cells and biosensors.(U.S. Patent Publications 20090242406, 20080253929, 20060292407 and U.S.Pat. Nos. 6,444,339 and 7,381,858).

In most electrochemical membrane applications, limiting current behavioris the source of concentration over-potential, which significantlylimits the ion/chemical transport through the membrane. As such, a needexists to develop a method for effectively reducing or preventinglimiting current behavior. A reduction in limiting current behaviorwould potentially enhance the electrochemical membrane performancesignificantly.

SUMMARY OF THE INVENTION

In part, the invention relates to a method of reducing or eliminatinglimiting-current behavior in the operation of electrochemical systems,in particular those with ion-selective membrane or electrochemicalelectrodes, by spatially reducing the convection near the membrane orthe electrode. The invention further relates to electrochemical systemsin which micropore arrays, microarrays or pillar arrays are used toreduce convection in comparison to conventional systems withoutmicroarrays, micropore arrays, or pillar arrays.

In one embodiment, the invention relates to an electrochemical systemcomprising a substrate, a plurality of microchannels fabricated ontosaid substrate, and a nanojunction connecting at least two of saidmicrochannels wherein at least a part of said substrate contains amicroarray, micropore array or a pillar array. In certain embodiments,the substrate contains a micropore array or a pillar array. In certainadditional embodiments, the substrate contains a pillar array at theanodic side of the nanojunction. In yet another aspect, the substratecontains a micropore array at the anodic side of the nanojunction. Thesystem can be one in which a microchannel can contain a fluid whereinwhen a current is applied, convection currents near said nanojunction isreduced when compared to convection currents near a nanojunction of sametype of electrochemical system without a pillar array or microporearray. The system can further comprise an ion-selective membrane betweensaid microchannels.

In additional aspects, the invention relates to a method of reducinglimiting current behavior across an ion-selective membrane in anelectrochemical system comprising providing a fabricated non-planarstructure on at least one side of the membrane and wherein theelectrochemical system comprises a substrate, wherein said substratecomprises a microchannel. Convection near said membrane can be reducedin comparison to having planar structures on both sides of the membrane.In one embodiment, the non-planar structure is a micro-array, a pillararray, or a micropore array. In additional aspects, the microchannel iscurved in shape, for example, the microchannel can be parabolic inshape. In some embodiments, the microchannel has a locus in proximity toa nanochannel.

In certain additional aspects, the invention is directed to a method ofreducing an ion depletion region in an electrochemical system comprisingproviding a fabricated non-planar structure on at least one side of anion-selective membrane wherein said non-planar structure reducesconvection near said membrane.

In yet another aspect, the invention is an electrochemical systemcomprising a substrate, a plurality of fluidic channels fabricated onsaid substrate, wherein at least two separate fluidic channels areconnected by a junction, wherein at least one part of said substratecontains a pillar array. In one embodiment, the fluidic channels aremicrochannels and/or the junction is a nanojunction. In yet anotheraspect, the junction can comprise an ion-selective membrane, forexample, a Nafion membrane.

In an additional embodiment, the invention is directed to anelectrochemical system comprising a substrate, a plurality of fluidicchannels fabricated on said substrate, wherein at least two separatefluidic channels are connected by a junction, wherein at least one partof said substrate contains more than one set of microarrays. In someembodiments, the microchannels are separated by a pillar-free gap, forexample having dimensions of about 25 to about 500 μm. The inventionalso encompasses the electrochemical system wherein a nanojunctionconnects said gap to a fluidic channel. In some embodiments when theelectrochemical system comprises a pillar array, the average diameter ofpillar is between about 0.1-1000 μm, or between about 1-100 μm, orbetween about 5-50 μm, between about 5-25 μm or between about 7-15 μm.In another aspect, the electrochemical system comprises a pillar arraywherein the average pillar height is between about 1-1000 μm, or betweenabout 1-500 μm, or between about 1-250 μm, or between about 5-100 μm, orbetween about 5-50 μm, or between about 5-25 μm. In certain embodiments,when the electrochemical system comprises a micropore array, the averagediameter of pillar is between about 0.1-500 μm or between about 1-50 μm,or between about 2-25 μm and/or wherein the pore depth is between about1-250 μm, or between about 2-200 μm, or between about 5-100 μm. In someembodiments, the substrate is made from polydimethylsiloxane.

In certain aspects, the invention is directed to electrochemical systemscomprising a structure selected from a pillar array or micropore arrayin proximity to an electrode or an ion-selective membrane such thatconvection is spatially limited and/or the zone of ion depletion isreduced in size. In additional aspects, the invention is directed to amethod of decreasing limiting current behavior or limiting convectionnear an electrode in an electrochemical system or enhancing electrodeperformance in an electrochemical system comprising providing an arrayof micropores or a pillar array in proximity to the electrode or to theion-selective membrane such that convection is spatially limited and/orthe zone of ion depletion is reduced.

The invention encompasses a method of decreasing limiting currentbehavior or limiting convection near an electrode in an electrochemicalsystem or enhancing electrode performance in an electrochemical systemcomprising providing an array of micropores or a pillar array inproximity to the electrode such that convection is spatially limitedand/or the zone of ion depletion is reduced. In certain additionalembodiments, the electrochemical system comprises a support and anelectrode. In certain embodiments, the micropore array or pillar arrayis positioned over the electrode. In some embodiments, the methodcomprises an array of micropores located over the electrode. Themicropore array can, for example, be fabricated from a photoresistpolymer, such as SU8. In some embodiments, the pore diameter is betweenabout 10 and about 100 μm, or between about 10 and about 40 μm. Incertain embodiments, the average distance between the pores of saidarray of micropores is between about 2 and about 100 μm or between about20 and about 50 μm. In certain aspects, the depth of each of the poresof said array is between about 10 and 100 um. In some embodiments, themethod comprises an electrochemical system comprising a microchannelwherein the microchannel is fabricated from PDMS.

In yet an additional aspect, the invention includes an electrochemicalsystem comprising a micropore electrode seated in a reservoir, anelectrolyte, a substrate and a support, wherein said substrate comprisesone or more microchannels and wherein said micropore electrode comprisesan electrode and a micropore array, wherein said micropore array isplaced over said electrode. In some embodiments, the micropore array isfabricated from a photoresist polymer, such as SU8. In some embodiments,the pore diameter is between about 10 and about 100 μm, or between about10 and about 40 μm. In certain embodiments, the average distance betweenthe pores of said array of micropores is between about 2 and about 100μm or between about 20 and about 50 μm. In certain aspects, the depth ofeach of the pores of said array is between about 10 and about 100 um. Ina further embodiment, the invention is an electrochemical systemcomprising an electrode seated in a reservoir, a pillar array placedover said electrode, an electrolyte, a substrate and a support, whereinsaid substrate comprises one or more microchannels. In certain aspects,the average diameter of a pillar is between about 0.1-1000 μm, orbetween about 1-100 μm, or between about 5-50 μm, between about 5-25 μmor between about 7-15 μm. In another the electrochemical systemcomprises a pillar array wherein the average pillar height is betweenabout 1-1000 μm, or between about 1-500 μm, or between about 1-250 μm,or between about 5-100 μm, or between about 5-50 μm, or between about5-25 μm. Also included in the present invention are methods of limitingconvection near an electrode in an electrochemical system or enhancingelectrode performance in an electrochemical system comprising providingan electrochemical system described herein.

The invention also encompasses a method of limiting convection near anelectrode in an electrochemical system or enhancing electrodeperformance comprising providing non-planar structures suspended overthe electrode, wherein said suspended non-planar structures are not indirect contact with the electrode therefore maintaining the activesurface area of the electrode. In certain aspects, the non-planarstructures are an array of micropores. In certain additional aspects,the non-planar structures are an array of pillars.

The invention also includes methods of increasing the electrochemicalefficiency of an electrode or an ion-selective membrane comprisingdecreasing the spatial extent of the ion depleted region. The spatialextent of the ion depleted region can be reduced by incorporating in theelectrochemical system a structure described herein (for example, apillar array or a micropore array). As described in more detail below,the smaller the ion depletion region, the greater the electrochemicalefficiency of the system.

Also encompassed in the present invention are fuel cells and/orbiosensors comprising an electrochemical system described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A: Schematic diagram of microchannel/nanojunction hybrid systemwith pillars. PDMS substrate having microchannels and verticalnanoporous junctions bonded with a glass support.

FIG. 1B: Sequences of fluorescent images of depletion boundary nearnanojunctions for nonpillar and pillar array (pillar-all) system. 30V(left)-15V (right) was applied at the anodic side microchannel.

FIG. 2A: Plots for current drop through nanojunction as a function oftime (fixed voltage) in non-pillar system.

FIG. 2B: Plots for current drop through nanojunction as a function oftime (fixed voltage) in pillar array system showing significantdifference in current level.

FIG. 3A: I-V plot for with/without pillars showingOhmic/limiting/over-limiting current. 1 mM phosphate buffer solution wasused. Also the current behavior in the separated pillar system (50 μmand 100 μm) was shown.

FIG. 3B: Sequence of fluorescent images of growing depletion zone fornon-pillar, 50 μm separated pillar and pillar array system.

FIG. 4: I-V plot for pillar all and non-pillar systems.

FIG. 5A: Pillar array, narrow-channel and non-pillar systems showing theunstable current behavior in a non-pillar system. I-V curve of narrowedchannel had moderate slope compared to pillar and non-pillar system.

FIG. 5B: Snapshots of fluorescent images of depletion zone growth.

FIG. 6A: Schematic diagram of the micro-hole structure that can hold thedepletion zone on top of the electrode.

FIG. 6B: Fabricated micro-hole structure on top of the electrodes.

FIG. 6C: Comparison of the current behavior at 5V applying voltage withand without the micro-hole structure.

FIGS. 7A-7C: Difference between planar vs. microelectrode. (A) In theplanar electrode, depletion zone is formed in a planar fashion, and thediffusion of charge carriers toward the electrode is inefficient. As aresult, the concentration overpotential is significant. (B) and (C), inthe line-patterned or dot-patterned microelectrodes, the diffusionalpattern is either 2D or 3D, which is more efficient. Depletion zone islimited in length, which leads to smaller concentration overpotentials.

FIGS. 8A and 8B: Exploded schematic view of micro-pore electrode andsnapshots of planar (bare) (B) and micro-pore electrodes (A).

FIGS. 9A and 9B: Close-up (5× magnification) snapshots of micro-poreelectrode for pore diameter=20 μm, depth=15 μm and pore center-to-centerdistance=(A) 30 μm & (B) 40 μm.

FIG. 10: Schematic of electrode concentration polarization for (a)micro-pore and (b) planar electrodes.

FIG. 11: Normalized steady state current plot with differentconcentrations of electrolyte. Micropore electrodes specifications arepore diameter=20 μm, center-to-center spacing=30 μm and depth (a)=15 μmdenoted as microelectrode 1 and (b)=30 μm denoted as microelectrode 2.The top line of the graph corresponds to microelectrode 2, the middleline corresponds to microelectrode 1 and the bottom line corresponds tothe planar electrode.

FIG. 12: Normalized average fluorescence intensity plots for differentelectrodes at a height of z=50 μm w.r.t. Au electrode surface. The smallinsert presents a typical confocal image with size 1024×1024 pixels andthe small red square (region of interest, ROI, is a 270 by 270 μmsquare) is the post-processing windows for average intensity (y-axis)plot. Fluorescent images was recorded by Zeiss LSM 510 laser scanningconfocal microscope. All images were captured at steady state currentcondition (t>600 s) and normalized respect to its initial conditions att=0. Working electrolyte for depletion observation is 0.3 mM Potassium(IV) Hexachloroiridate in 0.1 M Potassium Nitride with Alexa Fluor 488fluorescence in volume ratio of 1/1000. Micropore electrodesspecifications are pore diameter=20 μm, center-to-center spacing=30 μmand depth (a)=15 μm denoted as microelectrode 1 and (b)=30 μm denoted asmicroelectrode 2.

FIGS. 13A-13C: Micro-pore Electrode Arrays (MEA) (A) Traditional Model:in the diffusive model of MEA, enhanced performance of MEA is understoodas a result of dissimilar diffusive transport (1D vs. 3D) at the mouthof the ‘shallow’ recessed electrode hole. (B) our Model: In reality,there will always be an electrokinetic flow (surface-driven, amplifiedwithin the depletion zone), which could lead to a fast, circulating flowwithin the structure. This could significantly enhance the masstransport, and therefore limit the propagation of the electrodepolarization. (C) idealized electrode ‘silencer’ structure: If a fluidrestricting structure is placed (without blocking active electrodesurface), one could still achieve the same effect of ‘arresting’ thepropagation of depletion zone (concentration polarization), as in MEAs((A) & (B)). The key difference is that the active surface area ofelectrode is maintained, therefore achieving the highest net currentflowing through the system.

FIG. 14: Simple electrochemical system schematic and snapshot of testingvehicles. Microhole electrode was created by patterning (viaphotolithography process) SU8 micro-pore structure over Au planarelectrode. The test vehicle was completed by covering the electrodepyrex glass with polydimethysiloxane (PDMS) microfluidic channel with auniform depth of 100 μm. The connecting channel is 500 μm in width andthe electrode chamber is dia.=2.2 mm.

FIG. 15: Chronoamperotmetry responses of different electrode systems forredox reduction process of Potassium (IV) Hexachloroiridate (IV) in 0.1M Potassium Nitrite. Three different systems were compared: Planarelectrode: bare electrode of 1 mm radius, Microhole electrode 1:micro-pore electrodes (1 mm total radius) with r=10 μm, d=30 μm anddepth=15 μm, Microhole electrode 3: micro-pore electrodes with r=10 μm,d=40 μm and depth=15 μm. Upon the application of reduction potential,the system current flow decayed as the electrolysis proceeds to depletethe electroactive species near the electrode surface. The net currentmeasurement show that, even though micropore electrode 1&3 hassignificantly smaller active electrode area than planar electrode, thecurrent reaches the steady states faster and maintains the same orhigher net current, in the long run (not shown in the diagram).

FIG. 16: Two-photon microscopy imaging of fluorescence intensity over aperiod of 50 seconds upon the application of external applied voltage of3V. Inserts are the snapshot fluorescent images at t=0 s (top rightfirst three images) and t=50 s (top left last three images). Thefluorescence intensity was recorded at a distance of z=50 um w.r.t. Auelectrode surface. The two-photon microscopy scanning spot is approx. 2um in diameter. Small red square on the images represents thepost-processing windows for fluorescence intensity analysis.Fluorescence was depleted at around 30 s at planar electrode as it wasgoverned by linear diffusion mechanism, meaning that the depletion layerwas expanded without any suppression. Conversely, fluorescence at bothmicro-pore electrodes was able to sustain over the entire experiment,indicating that the depletion layer has not yet expanded beyond theplane of interest and therefore it could be concluded that depletionlayer was well confined. Alexa Fluor 488 fluorescent dye withconcentration of 10 uL/mL was used with 1 mM DSP buffer electrolyte.Planar electrode: bare electrode with r=1 mm, Micro-pore electrode 1:r=10 μm, spacing=40 μm and thickness=15 μm, and Micro-pore electrode 3:r=5 μm, spacing=30 μm and thickness=15 μm.

FIGS. 17A and B: (A) Standard electrochemical three electrode measuringsystem and its electrical connection. This measuring configuration wasused in single electrode investigation and (B) Standard two-electrodemeasuring system connection. This measuring configuration was used in“system integration” investigations.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, “a” or “an” are taken together to mean one or moreunless otherwise specified.

In part, the invention relates to a method of reducing or eliminatinglimiting-current behavior in the operation of electrochemical systems,in particular those with ion-selective membrane or electrochemicalelectrodes, by spatially reducing the convection near the membrane orelectrode. An electrochemical reaction is one that takes place at aninterface between the electrodes and the electrolyte. The inventionrelates to electrochemical systems in which non-planar structures suchas micropore arrays, microarrays or pillar arrays are used to reduceconvection in comparison to conventional systems without suchstructures. The reduction in convection results in a reduction in thesize or extent of the zone of ion depletion. The terms “zone of iondepletion”, “ion-depleted region,” “zone of depletion,” and “depletionzone” are used interchangeably herein. The zone of ion depletion is thearea in proximity to the electrode surface or the membrane where theconcentration of ions is depleted or the region of interest where theelectro-active ions have been consumed. In the case of the pillar arraylocated on the anodic side of a nanojunction connecting parallelmicrochannels such as described in the examples section, the zone ofdepletion can, for example, be confined to the distance set by thedistance between the nanojunction and the pillars. In the case of amicropore array set over an electrode as described in the examplesbelow, the zone of depletion can be confined within the microporestructure.

While the local convection could be promoting over-limiting current,strong circulatory vortices may be defining the extent of ion-depletedregion. Without being bound by any particular theory, it is postulatedthat limiting the size of circulatory flow would lead to a smaller iondepletion region, and the effective reduction or elimination of limitingcurrent behavior. As such, the invention is generally applicable toengineering strategies for electrodes and fuel cells in order to reducecurrent limitation posed by concentration overpotentials andconcentration changes near such electrochemical systems. This conceptcan be demonstrated by performing experiments in microfluidic systems.In the microfluidic channel, one can control the degree of convectionallowed by fabricating designed structures such as pillar arrays ormicropore arrays in order to localize the convective flow in the middleof microchannels, such as shown in FIG. 1A. Since the strong convectiveflow is generated inside the ion depletion zone, the pillars can bepatterned at the anodic side of microchannel.

Ion-selective or ion-exchange membrane refers to membranes that allowthe passage of the ions, while substantially maintaining the integritybetween the contents separated by the membrane. The particular materialselected for membrane can be changed for the electrode materialsselected and the desired rate of exchange of ions. Examples ofion-selective membranes include high aspect ratio ion-selectivemembranes made from polytetrafluroethylenes, perfluorosulfonates,polyphosphazenes, polybenzimidazoles, poly-zirconia,polyethyleneimine-poly(acrylic acid), poly(ethylene oxide)-poly(acrylicacid) and non-fluorinated hydrocarbon polymers. A preferred membrane isselected from Nafion, CMI 7000, Membranes International C/R, CMB andCCG-F from Ameridia, AM-1, AM-3 and AM-X and PC-200D.

In one embodiment, a device may contain a plurality of high aspectratio, ion selective membranes. In one embodiment a plurality ofhigh-aspect ratio ion selective membranes can be fabricated on a chip.In one embodiment fabrication of a plurality of high-aspect ratio ionselective membranes may be done using multi-blade fabrication. In oneembodiment multi-blade fabrication can be used for commercialization ofa device containing a self-sealed membrane. In one embodiment aself-sealed membrane refers to the high aspect ratio ion selectivemembrane. In one embodiment “self-sealed” means that after infiltration,or passage or filling of the trench or the gap, or the scratch made bythe blade with a liquid polymer solution, unbending the chip andsolidifying the polymer causes a self-sealing process of the scratch orthe gap or the trench by the polymer.

In one embodiment, multi-blade fabrication can be used for massiveparallelization of the membrane or the device fabrication process. Inone embodiment, multi-blade fabrication can be used to make a pluralityof membranes in parallel. In one embodiment, multi-blade fabricationrenders the fabrication process fast. In one embodiment, multi-bladefabrication renders the device a low-cost device. In one embodimentmulti-blade fabrication is combined with multi-syringe ormulti-dispenser system that enables parallel injection of liquid polymerto all trenches or cuts made by the multiple blades. In one embodiment,the multi-blade fabrication technique is part of an automatedfabrication technique, in which all steps of forming the high aspectratio ion selective membranes are automated, and all steps are performedin parallel on many channels or on many device parts or on many devices.In one embodiment, such automation enables mass production of devices,low cost, high yield and reproducibility of device properties. In oneembodiment parallel multi-blade fabrication facilitates quality controland reliability measurements to be done on selected devices. In oneembodiment, multi-blade fabrication and/or automation of the process isachieved using computers, computer programs, robotics or a combinationthereof. In one embodiment, the number of high aspect ratio ionselective membranes produced is equal to the number of channelsdescribed herein above. In one embodiment the number of high aspectratio ion selective membranes produced is greater than the number ofchannels described herein above. In one embodiment, the number of highaspect ratio ion selective membranes produced is smaller than the numberof channels described herein above. In one embodiment, the number ofhigh aspect ratio ion selective membranes produced is more than 5, or,in other embodiments, more than 10, 96, 100, 384, 1,000, 1,536, 10,000,100,000 or 1,000,000 channels, or in any number desired to suit aparticular purpose.

A pillar array has a plurality of pillars. In some embodiments, thepillar diameter is between about 0.1-1000 μm, preferably between about1-100 μm, more preferably between about 5-50 μm, more preferably betweenabout 5-25 μm, more preferably between about 7-15 μm. In someembodiments, the average pillar size was between about 1-1000 μm. Insome embodiments, the average pillar size was between about 1-500 μm. Insome embodiments, the average pillar size was between about 1-250 μm. Insome embodiments, the average pillar size was between about 5-100 μm. Insome embodiments, the average pillar size was between about 5-50 μm. Insome embodiments, the average pillar size was between about 5-25 μm. Insome embodiments the average distance between the pillars in the pillararray are between about 0.1-500 μm. In some embodiments the averagedistance between the pillars in the pillar array are between about 1-100μm. In some embodiments the average distance between the pillars in thepillar array are between about 5-50 μm. In some embodiments the averagedistance between the pillars in the pillar array are between about 5-25μm.

A micropore array has a plurality of micropores. In some embodiments,the pore diameter is between about 0.1-500 μm. In certain embodiments,the pore diameter is between about 10-100 μm. In some embodiments, thepore diameter is between about 1-50 μm. In additional aspects, the porediameter is between about 10-40 μm. In some embodiments, the porediameter is between about 2-25 μm. In some embodiments, the porediameter is between about 5-20 μm. In some embodiments, the pore depthis between about 0.1-500 μm. In some embodiments, the pore depth isbetween about 1-250 μm. In some embodiments, the pore depth is betweenabout 2-200 μm. In some embodiments, the pore depth is between about5-100 μm. In additional aspects, the pore depth is between about 10-100μm. In some embodiments the average distance between the pores in themicropore array are between about 1-500 μm. In some embodiments, thedistance between the pores in the micropore array are between about2-250 μm. In some embodiments, the distance between the pores in themicropore array are between about 2-100 μm. In some embodiments, thedistance between the pores in the micropore array are between about 5-75μm. In some embodiments, the distance between the pores in the microporearray are between about 10-50 μm. In additional embodiments, thedistance between the pores is between about 20-50 μm.

In some embodiments, more than one set of micropores/pillar arrays arepresent, where the gap between the sets of arrays is at least about 50%greater than the average distance between the individual pillars/poresin the array. In some embodiments, the gap between the arrays is betweenabout 10-1000 μm. In some embodiments, the gap between the arrays isbetween about 25-500 μm. In some embodiments, the gap between the arraysis between about 25-200 μm.

In one embodiment, the width of the microchannel is between about0.1-500 μm, and in one embodiment, the width of the channel is betweenabout 5-200 μm. In some embodiments, the width of the channel is betweenabout 20-1200 μm. In some embodiments the width of the channel isbetween about 50 and 500 μm. In some embodiments the width of thechannel is between about 50 and 250 μm.

In some embodiments, the depth of the microchannel is between about0.5-200 μm, and in some embodiments, the depth of the channel is betweenabout 5-150 μm. In some embodiments, the depth of the channel is betweenabout 5-100 μm. In some embodiments, the depth of the channel is betweenabout 5-50 μm. In some embodiments, the depth of the channel is betweenabout 5-25 μm. In some embodiments, the depth of the channel is betweenabout 10-25 μm. In some embodiments, the depth of the channel is betweenabout 10-20 μm.

In some embodiments, the ion-selective membrane has a width of betweenabout 0.01-100 μm, and in some embodiments, the width of theion-selective membrane is between about 1-10 μm. In some embodiments,the ion-selective membrane has a width of between about 100-500 nm. Insome embodiments, the ion-selective membrane has a depth of betweenabout 0.01-3000 μm, and in some embodiments, the depth of theion-selective membrane is between about 10-500 μm and in someembodiments, the depth of the ion-selective membrane is between about100-1000 μm. In some embodiments, the ion-selective membrane has a depthof between about 500-1100 μm. In some embodiments, the membrane iscation selective. In some embodiments, the membrane is anion selective.

In some embodiments, the fluidic chip comprises a silicon polymer,preferably, polydimethylsiloxane (PDMS). In some embodiments, thefluidic chip has a hydrophobic surface. In some embodiments, the fluidicchip comprises an elastomeric polymer. The elastomeric polymer can be asilicone elastomeric polymer. The elastomeric polymer can be solidifiedby curing. In some embodiments, the elastomeric polymer can be treatedwith high intensity oxygen or air plasma to permit bonding to thecompatible polymeric or non-polymeric media. The polymeric andnon-polymeric media can be glass, silicon, silicon oxide, quartz,silicon nitride, polyethylene, polystyrene, glassy carbon, or epoxypolymers.

Construction of the microchannels may be accomplished according to, orbased upon any method known in the art, for example, as described in Z.N. Yu, P. Deshpande, W. Wu, J. Wang and S. Y. Chou, Appl. Phys. Lett.,77 (7), 927 (2000); S. Y. Chou, P. R. Krauss, and P. J. Renstrom, Appl.Phys. Lett., 67 (21), 3114 (1995); Stephen Y. Chou, Peter R. Krauss andPreston J. Renstrom, Science, 272, 85 (1996), U.S. Patent Publication20090242406, and U.S. Pat. No. 5,772,905, which are incorporated hereinby reference. In one embodiment, the microchannels can be formed byimprint lithography, interference lithography, self-assembled copolymerpattern transfer, spin coating, electron beam lithography, focused ionbeam milling, photolithography, reactive ion-etching, wet-etching,plasma-enhanced chemical vapor deposition, electron beam evaporation,sputter deposition, stamping, molding scanning probe techniques andcombinations thereof. In some embodiments, the methods for preparationof the devices of this invention may comprise or be modifications ofAstorga-Wells J., et al, Analytical Chemistry, 75: 5207-5212 (2003); orJoensson, M. et al, Proceedings of the MicroTAS 2006 Symposium, TokyoJapan, Vol. 1, pp. 606-608. Alternatively, other conventional methodscan be used to form the microchannels. In one embodiment, themicrochannels are formed as described in J. Han, H. G. Craighead, J.Vac. Sci. Technol., A 17, 2142-2147 (1999) and J. Han, H. G. Craighead,Science, 288, 1026-1029 (2000), hereby incorporated fully herein byreference.

In one embodiment, a series of reactive ion etchings are conducted,after which nano- or micro-channels are patterned with standardlithography tools. In one embodiment, the etchings are conducted with aparticular geometry, which, in another embodiment, determines theinterface between the microchannels, and/or nanochannels. In oneembodiment, etchings, which create the microchannels, are performedparallel to the plane in which etchings for the nanochannels arecreated. In another embodiment, additional etching, such as, forexample, and in one embodiment, KOH etching is used, to produceadditional structures in the device, such as, for example, for creatingloading holes.

In one embodiment, an interface region is constructed which connects thechannels on the chip, for example two microchannels. In one embodiment,diffraction gradient lithography (DGL) is used to form a gradientinterface between the channels of this invention, where desired. In oneembodiment, the gradient interface region may regulate flow through theconcentrator, or in another embodiment, regulate the space charge layerformed in the microchannel, which, in another embodiment, may bereflected in the strength of electric field, or in another embodiment,the voltage needed to generate the space charge layer in themicrochannel. In some embodiments, the ion-selective membrane ispositioned at such an interface.

In another embodiment, the device may contain at least two pairs ofelectrodes, each providing an electric field in different directions. Inone embodiment, field contacts can be used to independently modulate thedirection and amplitudes of the electric fields to, in one embodiment,orient the space charge layer, or a combination thereof.

In some examples, the electrochemical systems and methods describedherein can be used in devices and applications which are associated withelectrode polarization, such as concentration polarization. Non-limitingexamples of such devices and applications include solid oxide fuel cells(SOFCs), lithium ion batteries, biosensors and dielectricspectroscopy-based sensors.

By limiting the lateral dimension of the local circulating flow near amembrane or electrode, a reduction in the limiting current behavior canbe achieved resulting in enhanced ion transport. In some embodiments,the current behavior can be limited by addition of the structuralfeature described herein without modifying the chemistry or physics ofthe electrode or membrane operation.

EXAMPLES Example 1 Electrochemical System Composed of Two ParallelMicrochannels Connected by Nanochannels (or a Nanoporous Membrane)

Compared to the classical membrane geometry that blocked a straightchannel, fluid flow in one embodiment is not blocked by the membrane butrather flows along it. The polydimethylsiloxane (PDMS) microfluidicchips were fabricated with perm-selective nanojunctions using thepreviously published methods. (S. J. Kim, and J. Han, Anal. Chem. 80,3507 (2008)). The anodic and cathodic microchannel had the dimension ofabout 100 μm width×15 μm depth. Pillar arrays were fabricated at theanodic side of microchannel and they had the size of about 10 μmdiameter (about 15 μm height). The gap between each pillar was about 10μm. Separated pillar systems had either about 100 μm or about 50 μmdistance between two groups of pillar arrays. A Nafion (sulfonatedtetrafluoroethylene based fluoropolymer-copolymer) nanojunction wasinfiltrated at the center of pillar structures.

1 mM of potassium phosphate dibasic solution (pH=8.4) was used as mainbuffer solution and it contained 1 μg/ml of FITC for fluorescenttracking. All the flow patterns were imaged with an invertedfluorescence microscope (Olympus, IX-51) and a CCD camera (SensiCam,Cooke corp.). Sequences of images were analyzed by Image Pro Plus 5.0(Media Cybernetics Inc.). A DC power supply (Keithley 236 source measureunit (SMU)) was used to apply electrical potential to each reservoirthrough a voltage divider. As shown in FIG. 1A, Ag/AgCl electrodes (A-MSystems, Inc.) were placed into each reservoir for proper electricalconnections. At the same time, the ionic current through nanojunctionswas also measured by Keithley 236 SMU. The sequence of current wasanalyzed by commercial software, LabView 8.2 (National Instrument). Forthe ionic current with constant applied voltages, the same voltages wereapplied at the anodic sides, while the cathodic sides were electricallygrounded. The currents were measured at every 0.1 second. For measuringOhmic/limiting/over-limiting current, voltages at the anodic sides wereramped up from 0V to 80V at the rate of 0.2V/30 sec.

From fluorescent images in FIG. 1B, the fluid vortex generated in thepillar-array system was shown to be mostly confined between pillars,while the vortex (related to the size of depletion zone) continuouslyexpanded their size in non-pillar system. In the pillar-array systemwhere laterally limited vortex was observed, the following effects wereobserved; 1) the concentration gradient was maintained at higher levelso that diffusive transport of ion is promoted, while it quickly reachesnear zero with rapidly expanding depletion region in a nonpillar system;and 2) shorter diffusion length (lower overall electrical resistance)results in higher electric field, which is the driving force of driftion transport, inside the depletion zone. These attributions wereconfirmed by measuring ionic currents through the nanojunctions asfunctions of both time and applied voltage as shown in FIGS. 2A, 2B andFIG. 3A. In the first case (FIG. 2A), the constant voltages weresuddenly applied at both anodic side microchannels, while the cathodicmicrochannels were electrically grounded. At low voltages (e.g., 1 V inFIGS. 2A and 2B), time-independent currents were observed in bothsystems because the ion current did not reach the limiting currentregime yet at such low voltages. While the non-pillar channel showsprecipitous drop in ionic current due to the formation of ion depletionzone (i.e., continuously increasing electrical resistance) at higherapplied voltages as shown in FIG. 2A, ion currents in the pillar-allsystem recover from the initial drop and increased up to −70% of theoriginal level as shown in FIG. 2B, demonstrating an effectiveelimination of current suppression caused by depletion. The initialcurrent drop in pillar-array system was due to the formation of iondepletion zone within the interpillar distance.

Example 2 Limiting/Over-Limiting Current Behavior

The current-voltage characteristics across the nanojunction weremeasured to characterize their detailed limiting current behaviors. Inaddition to the pillar and non-pillar systems shown in FIG. 1B, twoadditional systems were fabricated with a defined gap between pillararray and the nanojunction (50 μm and 100 μm) as shown in themicroscopic insets of FIG. 3A. The I-V traces for all systems largelyoverlapped in the sub-limiting current regime (less than 2V). Thefluorescent images less than 2V did not have any changes in all systemsas shown in FIG. 3B. However, critical differences were observed betweenpillar and non-pillar systems, in that (1) the limiting current behaviorwas almost eliminated in the pillar-array system, (2) overall currentlevel was higher in any kind of pillar system than in the non-pillarsystem. Confined vortex leads to much shorter diffusion lengths in anypillar systems along over-limiting current regimes as confirmed by thefluorescent images (FIG. 3B), leading to higher diffusive and driftion-flux. These effects were seen more clearly in the twoseparated-pillar systems. Due to the space between the pillar arrays,the depletion zone initially formed just as in non-pillar systems. Atthis moment, the limiting current behavior was observed as usual. Thiscan also be seen by the fact that the I-V traces for distanced-pillarand non-pillar systems are overlapping in the limiting current regime(at 5 nA in the voltage range of 2V-4.5V). As the voltage increasedabove the limiting regime, however, the lateral size of the fluid vortexare essentially determined by the distance between the pillar array andthe nanojunction, since the depletion boundary is ‘pinned’ at the pillararray and cannot expand any more as shown in FIG. 3B. In suchconditions, the resulting (over-limiting) current levels were muchhigher than that of non-pillar system. For example, the lengths ofdepletion boundary were approximately 250 μm (non-pillar system) and 60μm (pillar-all system) at 17V (FIG. 3B) and their current values were 25nA (non-pillar system) and 100 nA (pillar-array system) (FIG. 3A).Importantly, the current level of each system was highly correlated withthe distance between pillar structures as observed in two separatedpillar systems. Since the amplified electrokinetic vortical flows weredominant inside the depletion zone, shorter gap (50 μm) enforcedrelatively higher electric field and higher concentration gradient thanwider gap (100 μm). As a result of this, the over-limiting current of 50μm-separated pillar system had increased faster than 100 μm separatedsystem. The inset plot in FIG. 3A shows the distinct difference ofover-limiting current behaviors between each system (wider x- and y-axisrange).

From the experimental results of FIG. 3A, one can conclude that theextent of over-limiting current behavior is related to the lateral sizeof the depletion zone, which in turn is determined by the convectionprofile. In a pillar system, depletion region does not expand much morethan the distance from the nanojunction to the nearest pillar, leadingto a near-complete elimination of limiting current behavior. In thenon-pillar and 50 μm/100 μm separated pillar systems, depletion regioncan start to expand when the bias reaches the critical point (2V, nearthe onset of the limiting current). In the 50 μm and 100 μm separatedpillar systems, depletion region was ultimately confined to the distanceset by the distance between the nanojunction and the pillars. Theshorter this distance is, the lower overall resistance in theoverlimiting current regime. This is reflected in the different slopesin FIG. 3A. However, doubled distance did not exactly impose doubledresistance (slope of 1-V curve) because the concentration distributioninside the depletion zone could be altered by addition parameters suchas the degree of convection and an EOF through the microchannel, etc.

Example 3 Confining Convection Using Narrow Microchannel

In another scenario, we narrowed down the microchannel width of thenon-pillar system only around the nanojunction as shown in inset of FIG.5A. The narrow region has only 10 μm width (200 μm length). Due to highflow resistance, the current through the groove decreased such that adirect comparison to pillar system could not be made. Thus, in the Ohmiccurrent region<2V), I-V curve of narrowed channel had moderate slopecompared to pillar and non-pillar system as shown in FIG. 5A. However,the groove was able to limit the convection and hold the depletion zoneinto the narrowed region as shown in FIG. 5B. The density of electricfield inside the narrowed region is geometrically 10 times larger thanoutside the region. Furthermore, the electric field inside the depletionzone can be amplified more than 30 times greater than one outside thedepletion zone. These combined effects gave an extremely high gradientof electric field at the narrowed region interfaces and would play asignificant role of holding the depletion zone inside the narrowedregion. With aforementioned consideration, the limiting current behavioralmost disappeared and the current level was much higher than non-pillarsystem as shown in FIG. 5A.

Example 4 Enhancing the Stability of Electrochemical Membrane Systems

The pinned depletion boundary also significantly affects the stabilitiesof measured current behavior (current fluctuation) which is thedeterministic factor for the performance of the ICP relatedapplications. As shown in FIG. 4, the current plot of non-pillar systemhad large standard deviation, while most of the values were overlappedin the case of pillar-all system. In a non-pillar system the depletionregion is constantly expanding, changing the system resistance,concentration profile and (electroosmotic) flow dynamically. While thiscould be the source of the instability and run-to-run variability, theconfinement of convection (and depletion zone) in pillar systemseliminates dynamical changes in resistance/current. Therefore, thedefined depletion zone would induce more stable performance of themembrane system.

Example 5 Limiting Convection Near Electrodes by Microstructures SingleElectrode Systems

Enhancing the performance of an electrochemical system can be achievedby optimizing various factors such as electrode/membranecharacteristics, used electrolyte and catalysts. For example, duringmicrofluidic fuel cell operation, a concentration boundary layer (ICPlayer) that depends on channel geometry and flow rate will develop inthe channel, starting at the leading edge of the electrode. Thus, themaximum current density of the fuel cell is determined by the rate ofthe convective/diffusive mass transport from the bulk to the surface ofthe electrode under the assumption of rapid electrochemical reaction.For limiting case, the oxidant/fuel concentration is zero at the entiresurface of the electrode, meaning high electrical resistance whichlowers the total performance of fuel cell. In order to enhancingdiffusive transport, one can use line electrodes, to which the diffusiveion transport occurs in two directions (2D diffusion-drift), or useplanar electrodes (1D diffusion-drift). In addition, point electrodes(3D diffusion-drift) can also be used. Planar type electrochemicalmembranes and electrodes generally increase the overall(membrane/electrode) surface area.

In one example, micro-pore structures on top of planar electrode withnon-conducting materials were fabricated, as shown in FIG. 6A. Simplephotoresist polymer such as SU8 (MicroChem Inc.) can be used forfabrication. This electrode system was similar to point electrodessystem. As shown in FIG. 6A, the micro-pore structure, which has thedimension of about 5-100 μm depth and about 5-20 μm diameter, can holdthe depletion zone thickness down to distance determined by thethickness (height) of the pore, by similar mechanism to the narrowmicrochannel case. FIG. 6B shows the fabrication of the micro-holestructure on top of the electrode. In some examples, the micro-holes areabout 20 μm in diameter, about 20 μm in distance between each hole andabout 50 μm in height. With this pattern on the electrodes, the currentbehaviors at 5V constantly applying voltage were measured by Keithley6514 electrometer as shown in FIG. 6C. With the structure, the initialdropped current, which was caused by initial formation of depletionzone, recovered quickly, while the current dropped and fluctuatedwithout the structure. This result showed that the current (powerdensity) in the system would be greatly enhanced because the highelectrical resistance due to the depletion zone could be confined withinthe micro-pore structure. This strategy can be applicable both to normalelectrodes and perm-selective membranes (such as electrolytes in fuelcell).

In another embodiment of this invention, non-conducting micro-porestructures were fabricated on top of a planar electrode, as shown inFIG. 8. Commercial photoresist polymer such as SU8 (MicroChem Inc.) withstandard lithography process can be used for micro-pore structurefabrication. FIG. 9A-9C shows the micro-pore electrode systems with apore diameter of 20 μm and center-to-center spacing of 30 μm and 40 μm.This electrode system was similar to a point electrode system in anarray formation. As shown in FIG. 10, the micro-pore structure which hasa range of size from 10-100 μm for depth, 5-20 μm for pore diameter and20-50 μm for pore spacing can suppress the growth of ion depletion (oralso known as electrode concentration polarization) and contain thisdepletion thickness relatively much thinner than planar electrode(electrode without structure). The degree of suppression is stronglydependent on the thickness (height) and spacing of the pore structure.The quantitative characterization of steady-state current response forplanar and two micro-pore electrodes is presented in FIG. 11. The planarelectrode was out-performed by the micro-pore electrodes as shown inFIG. 11 and these results were in good agreement with our earlierhypothesis. For a direct visualization, confocal imaging of fluorescenceintensity for different electrodes was recorded and illustrated in FIG.12. The fluorescence intensity was correlated to the concentration ofelectro-active species (i.e. ions) for the required electrical currentflow in an electrochemical system. The fluorescent images were capturedat a height of 50 μm with respect to the electrode surface after 600 supon the application of voltage for a steady state current flow. In FIG.12, it was clearly shown that for planar electrode without structure,the fluorescence was depleted to half of its initial concentration atsteady state current indicating that the ion depletion thickness wasgrown beyond 50 μm thick. However, with micro-pore structure thefluorescence concentration remained showing that the ion depletion wassuccessfully suppressed, and hence resulted in a higher current responseas shown in FIG. 11.

Overall, these sets of experimental observations and prior studiessuggest an emerging picture: Electrochemical efficiency of any electrode(or ion-selective membranes) would critically depend on the spatialextent of the depletion region, which forms a high-resistance barrier tothe system.

Adding a structural feature (which may be a ‘partially blocking’structure (as in FIGS. 13A and B)) would affect the local electrokineticfluid flow could bring about significant reduction in concentrationoverpotential (suppression of ion depletion thickness) and enhancementin membrane/electrode performance. In some embodiments, to further boostthe performance, some embodiments could be a suspended structuralfeature (FIG. 13C) without blocking the active electrode surfaces as inmicro-pore systems. This could lead to a significant boost in electrodeefficiency in various electrochemical energy devices, by addressing thekey limitation of the microelectrode arrays (MEA) application to energyapplications. The net result of the proposed structure in FIG. 13C wouldbe an additional mechanism to ‘arrest’ the propagation of depletionregion, which will lead to lower concentration overpotential and highercurrent density.

System Integration

In one example for a system-based performance evaluation, a completeelectrochemical system was set-up as shown in FIG. 14. Micro-poreelectrodes were fabricated using photolithography process as earliermentioned and the polymer-based micro-lengthscale channel networkstructure was fabricated with standard polydimethylsiloxane (PDMS)process. Micro-pore electrode was seated in a cylindrical reservoirlocated at each side of the system-of-interest (i.e. a microscaleconnecting channel in FIG. 14). Essentially, it is two disk-shapedmicro-pore electrodes connected by a microfluidic channel filled withelectrolyte, used for the impedance characterization. In FIGS. 15 & 16,we compared three cases; Planar electrode with no structural features,Microelectrode 1 with a denser array of micro-pores (dia.=10 μm,spacing=30 μm & depth=15 μm), Microelectrode 3 with an array ofmicro-pores (dia.=10 μm, spacing=40 μm & depth=15 μm). In all threecases, the original size of circular electrodes were the same, but inmicroelectrodes 1 & 3 an insulating film (SU-8) covers the majority ofthe electrode surface, creating a micro-pore array systems withrelatively high density (low spacing/dia. values). However, the active(open) area of electrode were diminished (to 46% for electrode 1 and 24%for electrode 3, respectively, of the planar electrode) from that ofplanar electrode system. When we measured the current in the systems inresponse to a DC bias, we found that the later, quasi-steady-state netcurrent values are quite similar, and at one time point theMicroelectrode 3 carries the largest current (FIG. 15). This result canalso be correlated to the propagation of diffusion layer (depletionzone) near the electrode, which were directly measured using the twophoton microscopy systems (FIG. 16), where it was shown that thedepletion region (monitored by dye tracer) were seen to grow more than50 μm in planar electrode within a minute from the biasing, but not inmicro-pore electrode systems.

All chemicals were purchased from Sigma Aldrich and used as receivedunless otherwise noted.

0.1 M Potassium Nitride (KNO₃) supporting electrolyte was prepared bydissolving 5.02 g KNO₃ salt in 50 mL de-ionized (DI) water. While thestock solutions 10 mM Potassium (IV) Hexachloroiridate (K₂IrCl₆) in 0.1M Potassium Nitride (KNO₃) were prepared by dissolving 483.1 mg K₂IrCl₆in 100 mL 0.1 M KNO₃. Several different dilute K₂IrCl₆ (0.1, 0.3, 0.5,1, 3, 5 mM) solutions were prepared by diluting 10 mM K₂IrCl₆ with 0.1 MKNO₃ at appropriate ratio.

Cyclic Voltammetry, and Chronoamperometry measurements were performedusing a Versastat 3 Potentiostat (V3 Studio Software, Princeton AppliedResearch) connected with a standard three-electrode cell for singleelectrode study, see FIG. 1. While a standard two-electrode measuringconfiguration was used in system integration study, see FIG. 2.

The three measuring electrodes are working electrode, counter electrodeand reference electrode (R. S. Rodgers, “Stalking the WildPotentiostat,” Today's Chemist at Work, June, 1995, p. 30. (V4#6)). Fourdifferent working electrodes were put on test and their surface areaswere 3.1416, 1.39, and 0.74 mm² for planar electrode, microholeelectrode 1 & 2 and microhole electrode 3 respectively. [Note: Planarelectrode: bare electrode of 1 mm radius, Microhole electrode 1:micro-pore electrodes (1 mm total radius) with r=10 μm, d=30 μm anddepth=15 μm, Microhole electrode 2: micro-pore electrodes (1 mm totalradius) with r=10 μm, d=30 μm and depth=30 μm and Microhole electrode 3:micro-pore electrodes with r=10 μm, d=40 μm and depth=15 μm.]

A platinum electrode with a large surface area (2.0±0.2 mm in diameterand ˜7 cm long) was employed as the counter electrode while thereference was an Ag/AgCl reference electrode (in 3 M NaCl/Saturated AgClfilling solution).

Cyclic voltametry (CV) measurements were recorded between 0.3 and 1V/Ag/AgCl at several scan rates 10, 50, 100, 500, 1000, 6000 mVs⁻¹,while all chronoamperometry (CA) measurements were recorded over a timeperiod of 600 s for a potential step from +0.9 to 0.3 V/Ag/AgCl. Thesteady current values reported in the text were an average currentresponse of the last 200 s of each CA test. All experiments were carriedout in quiescent solutions unless otherwise stated.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1-3. (canceled)
 4. A method of reducing limiting current behavior acrossan ion-selective membrane in an electrochemical system comprisingproviding a fabricated non-planar structure on at least one side of themembrane wherein convection near said membrane is reduced in comparisonto having planar structures on both sides of the membrane.
 5. The methodaccording to claim 4, wherein said non-planar structure is amicro-array. 6-12. (canceled)
 13. An electrochemical system comprising asubstrate, a plurality of fluidic channels fabricated on said substrate,wherein at least two separate fluidic channels are connected by ajunction, wherein at least one part of said substrate contains a pillararray. 14-17. (canceled)
 18. An electrochemical system comprising asubstrate, a plurality of fluidic channels fabricated on said substrate,wherein at least two separate fluidic channels are connected by ajunction, wherein at least one part of said substrate contains more thanone set of microarrays.
 19. The electrochemical system of claim 18,wherein at least two sets of microchannels are separated by apillar-free gap.
 20. The electrochemical system of claim 19, whereinsaid gap is between about 25 and 500 μm.
 21. (canceled)
 22. A fuel-cellsystem comprising an electrochemical system according to claim
 18. 23. Abiosensor comprising an electrochemical system according to claim 18.24. An electrochemical system comprising a pillar array wherein theaverage diameter of pillar is between about 0.1-1000 μm, or betweenabout 1-100 μm, or between about 5-50 μm, between about 5-25 μm orbetween about 7-15 μm.
 25. An electrochemical system according to claim18 comprising a pore array wherein the average diameter of pore isbetween about 0.1-500 μm or between about 1-50 μm, or between about 2-25μm.
 26. An electrochemical system according to claim 18 comprising apore array wherein the pore depth is between about 1-250 μm, or betweenabout 2-200 μm, or between about 5-100 μm.
 27. An electrochemical systemaccording to claim 18 comprising a pillar array wherein the averagepillar height is between about 1-1000 μm, or between about 1-500 μm, orbetween about 1-250 μm, or between about 5-100 μm, or between about 5-50μm, or between about 5-25 μm. 28-41. (canceled)
 42. An electrochemicalsystem comprising a micropore electrode seated in a reservoir, anelectrolyte, a substrate, and a support, wherein said substratecomprises one or more microchannels and wherein said micropore electrodecomprises an electrode and a micropore array, wherein said microporearray is placed over said electrode.
 43. The system of claim 42, whereinthe micropore array is fabricated from a photoresist polymer.
 44. Thesystem of claim 43, wherein the photoresist polymer is SUB.
 45. Thesystem of claim 42, wherein the pore diameter is between about 10 andabout 100 μm.
 46. (canceled)
 47. The system according to claim 42,wherein the average distance between the pores of said array ofmicropores is between about 2 and about 100 μm.
 48. (canceled)
 49. Thesystem according to claim 42, wherein the depth of each of the pores ofsaid array is between about 10 and about 100 um.
 50. (canceled)
 51. Thesystem according to claim 42, wherein the electrochemical systemscomprise at least two independent sets of micropore arrays. 52.(canceled)
 53. A method of reducing limiting current behavior orreducing convection near an electrode or enhancing electrode performancein an electrochemical system comprising providing an electrochemicalsystem according to claim
 41. 54. (canceled)
 55. A method of increasingelectrochemical efficiency of an electrode or an ion-selective membranein an electrochemical system comprising decreasing the spatial extent ofthe ion depleted region.
 56. The method of claim 55, wherein the spatialextent of the ion depleted region is decreased by incorporating intosaid system a pillar array or a micropore array in proximity to theelectrode or ion-selective membrane.
 57. (canceled)
 58. (canceled)